This study examines the role of activator/enhancer elements in the regulation of eukaryotic viral gene expression. In SV40, a pair of 72 bp tandem repeat sequences, located approximately 100-200 bp from the site of initiation of transcription, have been shown to be required for transcription. These sequences activate other genes placed 3' to the activators. Similar sequences have been identified in the long terminal repeat (LTR) of Murine sarcoma virus (MSV) and have been found to be capable of replacing the 72bp repeat sequences of SV40 to form a viable virus (SVrMSV). In this study, employing an assay for viral T-antigen expression as well as an in vitro assay for early gene expression involving the prokaryotic gene, chloramphenicol acetyl-transferase (CAT), host-specific gene activation was examined. Using identical promoter regions from SV40, the SV40 tandem repeats were found to be more active in monkey kidney cells, whereas the MSV repeats were more active in mouse cells. Therefore, at least some activators appear to function in a host specific manner.